In general, there are three types of communication systems; conventional, trunking, and cellular. Each system comprises a plurality of communication subscriber units, either mobile or portable (hereafter referred to as mobiles), a limited number of communication resources that are transceived by a repeater, and a communication resource allocator. However, each system operates in a somewhat different mode and supports different types of communications.
Conventional systems operate by dedicating a limited number of communication resources, often referred to as frequencies, to a plurality of mobiles over a large coverage area (20 to 40 square miles is typical). Thus, a dispatcher within the conventional system may communicate, on a dedicated frequency, with one or more of the mobiles currently operating at the dedicated frequency within the conventional coverage area. Such systems are useful for local entities, such as local law enforcement, which require few frequencies and a large but limited coverage area.
Trunked systems operate by sharing a limited number of frequencies amongst a large number of mobiles over a large coverage area. By dedicating one frequency within the trunked system as a control channel, a central controller is able to dynamically allocate the remaining frequencies so that multiple intra-group or dispatcher-to-group communications may take place concurrently. Such systems are useful to entities with a large number of mobiles, such as delivery or utility services, which require a large but limited coverage area.
Cellular systems operate by sharing a limited number of frequencies amongst a large number of mobiles over many small (2 square miles is typical), overlapping coverage areas. Each small coverage area, referred to as a cell, operates in a manner similar to a trunked system; there is a control channel dynamically allocating resources so that multiple communications may occur. These cells are networked together to allow mobiles to roam from cell to cell without loss of communications. By keeping the coverage area of each cell small, frequencies used within one cell may be reused in another cell which does not have an overlapping coverage area. Such systems are used in mobile car telephone systems, where efficient processing of one-to-one calls is required.
In general, each type of system described above is considered a stand-alone system. That is, they are not designed to communicate with one another. Their use of different frequencies and channel access protocols are examples of this incompatibility. However, as the communication business grows, many system users have changed from locally based users to more regional or nationally based users.
As result, once stand-alone systems are required to be linked together to support customer needs. This linking can be within similar systems, such as multiple conventional systems (See Maher et. al., U.S. Pat. No. 5,175,727. It is also possible to interconnect dissimilar systems, such as cellular and trunking systems (See Comroe U.S. Pat. Nos. 5,218,716; 5,313,654; 5,179,721; and 5,239,674. However, conventional systems, due to the dedicated nature of their communication resources, do not interface well with the shared communication resources of either a cellular or trunked system.
It is known for some communication units to operate within either type of system by changing their operating frequencies and channel access protocols. For instance, it is possible for a mobile based within a conventional system to operate within a trunked system. However, this ability does not allow another mobile or dispatcher within the conventional system to communicate with a mobile that has roamed into a trunked system. Therefore a need exists for a method which allows conventional systems to communicate with their associated mobiles that have roamed into, and are currently operating within, a trunked system.